Just wanted to tell you guys that we are using zimbra as our main mail server since today.
I'm fighting with users that are importing mailboxes of 5 gb (!!!). The pst importer is working nicely showing only warnings so far. It crashed one time without much explanation of why but could be a user error.
Users with big mailboxes are unable to delete emails, seems like email gets cached or smth. So i asked them to remove them locally and then re-import their pst.
Make sure that you make the reqd. registry change if you are importing big email attachments in the PSTs.
We've had people import 3-4 GB of PSTs without much problem.